Metal Finishing Guide Book

2012-2013

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coating materials and application methods AUTODEPOSITION OF ORGANIC COATINGS BY THOMAS C. JONES HENKEL SURFACE TECHNOLOGIES, MADISON HEIGHTS, MICH. Auto deposition of organic coatings refers to a chemical process for depositing paint coatings onto metal surfaces. This process resembles electrodeposition in that the objects to be coated are submerged in an aqueous bath containing ionized organic materials; however, in autodeposition, the coating is produced by a chemical reaction between the metal surface and the bath constituents as opposed to electrodeposition where the coating is produced by the electrolysis of water due to an imposed electric current. After autodeposition, the wet film is water rinsed and cured by application of heat to produce an inert, corrosion-resistant coating. COATING MECHANISM The first step in the coating mechanism for the autodeposition process is the chemical reaction between the metallic surface and the inorganic constituents of the coating bath. The coating deposition step of the autodeposition process involves the controlled destabilization of an aqueous polymer latex dispersion, which is negatively charged, by the positive ions generated at the surface of the metal by the inorganic chemical reaction. The components of an autodeposition coating bath include a weak acid (hydrofluoric acid, HF) in the range of 0.2% to 0.3% by volume, an anionically stabilized latex and pigment dispersion (latex/pigment), and chelated ferric ion in solution (FeF3). The total solids content of the bath is less than 10%, and the coating solution has the viscosity of water. The chemical reactions that result in an auto deposition coating are as follows. For iron dissolution, the major contributor is 2 Fe F3 + Fe 3 Fe+2 + 6 F-1, and a minor contributor is 2 HF + Fe Fe+2 + H2(g) +2 F-1; deposition occurs when Fe+2 + (latex/pigment) Fe(latex/pigment). Iron that is not entrapped in the wet film is converted to FeF4-1 by the addition of an oxidizing agent. Since FeF4-1 does not react with the anionically stabilized latex/pigment dispersion, the process is sludge-free. As the autodeposited film builds, the diffusion of reactants to the surface is slowed, and the rate of film deposition decreases. This self-limiting mechanism results in a final coating that is extremely uniform and conforms to the underlying surface. All areas exposed to the coating bath become coated. This feature 238

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